1. Field of the Invention
This invention relates generally to olefin polymerization catalysts and methods and, more particularly, this invention relates to a catalyst component and system useful in the polymerization of propylene.
2. Description of Related Art
It is well known that propylene can be polymerized using catalysts containing titanium trichloride in any of a variety of forms in combination with an organoaluminum compound. The polymer products, while predominantly crystalline, contain amorphous and low molecular weight components. In order to obtain a commercially useful polypropylene product, it is desirable to minimize levels of such amorphous and low molecular weight components in the product. The presence of these components leads to processing difficulties in the fabrication of products from the polymers, and the finished products are typically inferior in terms of their physical properties.
When some 1-olefin polymerization reactions are carried out in a hydrocarbon medium, low molecular weight material remains in the solvent and can readily be separated from the product. In bulk and gas phase processes, however, it is desirable to have catalysts which provide superior stereoregularity to eliminate the need to remove low molecular weight materials, and to minimize the presence of amorphous species in polypropylene products.
One approach to eliminating or reducing these problems is by means of modification of catalyst components. While a wide variety of catalyst modifiers are known in the art, care must be exercised in the selection and use of modifiers because reduction in levels of low molecular weight and amorphous by-products is typically accompanied by decreases in catalyst activity and polymerization rate. Accordingly, an effective catalyst modifier would be one which minimizes the production of amorphous and low molecular weight by-products without decreasing the activity of the catalyst system.
In some propylene polymerization plants, liquid propylene monomer is used to flush the catalyst into the reactor. Many high activity titanium catalysts can polymerize propylene even in the absence of a cocatalyst either cationically due to the presence of Lewis acid components or by a coordination mechanism due to the presence of aluminum alkyl residues from the catalyst preparation procedure. In an industrial continuous polymerization unit, this can lead to polymerization in the feed lines which can result in plugging and other catalyst feed problems. Thus, it would be desirable to provide a catalyst system with which polymerization in feed lines is inhibited prior to introduction to the reaction system.
One example of catalyst modification for the preparation of 1-olefin polymers, including propylene polymers, is disclosed in Staiger et al. U.S. Pat. No. 4,260,710 (Apr. 7, 1981). This patent discloses 1-olefin polymerization catalysts containing titanium trichloride which is milled together with an electron donor compound before use, alkyl aluminum compounds and certain phenolic compounds. It is disclosed that when the phenolic compounds are premixed with the alkyl aluminum compound and used as a catalyst modifier the polymerization rate of the catalyst is improved while minimizing the amount of alkane soluble by-products.
Another catalyst modification proposal is disclosed in Mueller-Tamm et al. U.S. Pat. No. 4,229,318 (Oct. 21, 1980) which discloses 1-olefin polymerization catalysts containing titanium trichloride, an alkyl aluminum compound and carboxylic acid esters. This patent discloses that an improved catalyst is produced by ballmilling a solid titanium trichloride compound with a carboxylic acid ester. Such modification is disclosed to improve the polymerization rate of the catalyst while maintaining the amount of alkane soluble by-products at a minimum. However, this method requires high capital investment in ballmilling equipment and the polymer produced with this catalyst is irregular in shape with a high concentration of fines, which causes synthesis and downstream problems.
In the Mueller-Tamm et al. patent, the titanium trichloride compound must be complexed with aluminum trichloride. In Staiger et al., the titanium trichloride may be free of aluminum trichloride but need not be.